High carbon steel formed bearing assembly

ABSTRACT

A roller bearing assembly and a method of forming a rolling bearing assembly. The bearing assembly includes a shell formed from a cold-rolled strip having a carbon content greater than 0.65%. In one aspect of the invention, the circumferential raceway has an as hardened surface hardness of at least 58 HRC without the application of a carburizing treatment to the formed shell. In another aspect of the invention, the cold-rolled strip has a carbon content greater than 0.65% and an ASTM grain size no. of 8 or finer. The circumferential raceway has an average surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell.

BACKGROUND

This invention relates to roller bearings with a roller race made from flat sheet metal. More particularly this invention relates to a roller bearing of the type including a roller race made from high carbon flat sheet metal.

One method of making a roller bearing is to form a race with end flanges from flat sheet metal. The sheet metal is subjected to a forming process to form the flat stock into a shell 12 which is closed or flanged at one end 14 and open at the opposite end 16. Examples of the forming process include deep drawing, stamping or cold forming. FIG. 1 is an illustrative forming process in which the flat sheet metal is drawn or formed between male and female dies 17 and 18, respectively. The forming process can occur in a single operation or a sequence of forming operations. The roller bearing 10 has a number of internal components (not shown) installed within cylindrical shell 12. The open end 16 is then bent over during final assembly to retain the internal components. Alternatively, the open end 16 is bent over, and then later the internal components are installed to create the final assembly.

The flat sheet metal used to form the shells has typically been made of low carbon steel, for example, a steel having a carbon content of less than 0.35%. To provide a sufficiently hard raceway, the shell is subjected to a process to add carbon to the surface, for example, carburizing, carbonitriding, or the like. All of these processes in which carbon is added to the surface are collectively referred to hereinafter as carburizing. One example of a carburizing process includes exposing the shell, either with or without the internal components installed, to a high carbon atmosphere at high temperatures, for example, a temperature greater than 850° C., for an extended period of time such that carbon from the atmosphere diffuses into the shell. The process produces a high-carbon, hardened surface layer, while the core of the shell, between the hardened surfaces, remains a lower carbon, softer area. The process can be expensive and time consuming, particularly when a deeper hardened surface layer is desired.

U.S. Pat. No. 6,682,227 discloses a bearing assembly manufactured from a cold-rolled strip wherein the carbon content is between 0.30-0.55%. After the shell is drawn, the bearing is still subjected to a carburizing treatment wherein carbon is added to the surface of the bearing. The 6,682,227 patent describes a process in which the bearing is maintained in a high carbon atmosphere at a temperature of approximately 850° C. for 25 minutes and then quenched. The bearing assembly is then subjected to a tempering process wherein the assembly is heated to approximately 170-200° C. for a certain time and then cooled. While the duration of carburizing may be reduced, the process is still required, thereby still requiring the provision of a high carbon atmosphere at high temperature for some time.

Another potential drawback for bearing shells formed from lower carbon flat sheet steel is the surface quality of the bearing raceway. Low carbon steels often experience microtearing during the forming process, thereby resulting in a surface with irregularities. In some applications, such irregularities are acceptable. However, in more precise applications, the shell must be subjected to a finishing process, for example, grinding, in order to smooth the bearing surface. Again, this process is expensive and time consuming.

The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

SUMMARY

The present invention provides a roller bearing assembly and a method of forming a rolling bearing assembly. The bearing assembly includes a shell formed from a cold-rolled strip having a carbon content greater than 0.65%. The formed shell has a circumferential raceway and a plurality of rolling elements are positioned in the shell. In one aspect of the invention, the circumferential raceway has an as hardened surface hardness of at least 58 HRC without carburizing of the formed shell. In another aspect of the invention, the cold-rolled strip has a carbon content greater than 0.65% and an ASTM grain size no. of 8 or finer. The circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell. In another aspect of the invention, the cold-rolled strip has a carbon content greater than 0.65% and a plain strain forming limit value (FL₀) greater than or equal to 0.25.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view in partial cross-section showing a roller bearing shell that is a first embodiment of the present invention formed via an illustrative forming process.

FIG. 2 is an isometric view of a formed bearing shell.

FIG. 3 is a cross sectional view along the line 3-3 in FIG. 2.

FIG. 4 is a cross sectional view similar to FIG. 3 showing the rolling elements positioned in the shell and the open end closed.

FIG. 5 is a cross sectional view similar to FIG. 3 showing an alternative configuration of the shell.

FIG. 6 is a sheet metal forming limit diagram (FLD) for an illustrative high carbon sheet steel for use in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Certain terminology, for example, “top”, “bottom”, “right”, “left”, “front”, “frontward”, “forward”, “back”, “rear” and “rearward”, is used in the following description for relative descriptive clarity only and is not intended to be limiting.

Referring to FIGS. 2-4, a roller bearing assembly 10 that is a first embodiment of the present invention is shown. The bearing assembly 10 includes a cylindrical shell 12 that is formed from a piece of flat stock. The shell 12 can have various configurations, for example, cylindrical, tapered, or a partial sphere, and is described herein as a cylinder for illustrative purposes only. The shell 12 is flanged at one end 14 and initially open at the opposite end 16. Referring to FIG. 5, the shell 12′ can have a closed end 14′ opposite the open end 16. The closed end 14′ may be provided with a raised portion 15. Various other configurations for the shell 12 may also be provided. The open end 16 often has a thinner cross section that is conventionally referred to as a lip 20. The shell 12 houses a number of internal components such as the rolling elements 22. Additional internal components may include a retainer cage to hold and guide the rolling elements, one or more seals, and less commonly, washers or rings for more specialized purposes. Once the internal components are installed, the lip 20 is bent, folded or otherwise turned to close the open end 16 of the shell 12. Alternatively, the lip 20 is bent, folded or otherwise turned to close the open end 16 of the shell 12 prior to installation of the internal components, this assembly being left to a later operational step.

Conventionally, shells formed through forming flat sheet stock, for example, through deep drawing, stamping or cold forming, have generally been limited to low carbon or medium carbon steels with a carbon content less than or equal to 0.55%.

The present inventors have found that the bearing shell 12 of the present invention may be formed from flat sheet stock that is a high carbon steel having a carbon content of 0.65% or greater. The material preferably has a chemical composition of: % carbon 0.65-0.90; % manganese 0.75-0.90; % chrome 0.15-0.25; % silicon 0.15-0.30; % phosphorus 0.025 max; % sulfur 0.025 max; and % aluminum 0.020-0.050. The preferred material also has an ASTM grain size no. of 8 or finer. For example, the inventors have produced the present bearing assembly 10 utilizing SAE-1074 carbon steel strip stock having an ASTM grain size no. of 8 or finer. While the inventors have found SAE-1074 carbon steel strip stock to be preferable, other high carbon steels may be utilized. It is preferred that such high carbon steels have an ASTM grain size no. of 8 or finer and a plain strain forming limit value (FL₀) greater than or equal to 0.25. As indicated in FIG. 6, the inventors have found that a material of this nature has an anisotropy value R that is close to 1.0 without working of the material, as is generally required in lower carbon steels with coarser grain sizes.

Roller bearing assemblies 10 manufactured using the preferred material provided a shell 12 having very good concentricity and roundness. The shell 12 also has a heat treated bearing hardness, for example, greater than 58 HRC, without any post-forming carburizing process due to the high carbon content. The formed shell 12 is typically tempered to increase the surface toughness, but generally does not require a carburizing treatment to harden the bearing surface. The formed shell 12 also has a substantially uniform carbon content through the cross-section of the shell 12. Furthermore, due to the fine grain structure, the formed shell 12 has a smooth surface without any additional finishing. For example, the formed shell 12 will preferably have an average surface roughness Ra of less than 18 microinchs and preferably less than 8 microinchs without any grinding or other supplemental finishing of the formed shell 12. 

1. A method of forming a rolling bearing component comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65%; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell; wherein the circumferential raceway has an as hardened surface hardness of at least 58 HRC without applying a carburizing treatment of the formed shell.
 2. The method according to claim 1 wherein the forming step includes deep drawing, stamping, cold forming or a combination thereof of the cold-rolled strip.
 3. The method according to claim 1 wherein the cold-rolled strip has an ASTM grain size no. of 8 or finer.
 4. The method according to claim 3 wherein the circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell.
 5. The method according to claim 1 wherein the cold-rolled strip has a plain strain forming limit value (FL₀) greater than or equal to 0.25.
 6. A method of forming a rolling bearing component comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65% and an ASTM grain size no. of 8 or finer; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell.
 7. The method according to claim 6 wherein the circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell.
 8. The method according to claim 6 wherein the forming step includes deep drawing, stamping, cold forming or a combination thereof of the cold-rolled strip.
 9. The method according to claim 6 wherein the circumferential raceway has an as hardened surface hardness of at least 58 HRC without applying a carburizing treatment of the formed shell.
 10. The method according to claim 6 wherein the cold-rolled strip has a plain strain forming limit value (FL₀) greater than or equal to 0.25.
 11. A method of forming a rolling bearing component comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65% and a plain strain forming limit value (FL₀) greater than or equal to 0.25; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell.
 12. The method according to claim 11 wherein the forming step includes deep drawing, stamping, cold forming or a combination thereof of the cold-rolled strip.
 13. The method according to claim 11 wherein the cold-rolled strip has an ASTM grain size no. of 8 or finer.
 14. The method according to claim 13 wherein the circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell.
 15. The method according to claim 11 wherein the circumferential raceway has an as hardened surface hardness of at least 58 HRC without applying a carburizing treatment of the formed shell.
 16. A rolling bearing assembly manufactured by the process comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65%; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell; wherein the circumferential raceway has an as hardened surface hardness of at least 58 HRC without applying a carburizing treatment of the formed shell.
 17. A rolling bearing according to claim 16 wherein the shell has a cross-section with a substantially uniform carbon content without applying a carburizing treatment to the formed shell.
 18. A rolling bearing assembly manufactured by the process comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65% and an ASTM grain size no. of 8 or finer; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell.
 19. The rolling bearing assembly of claim 17 wherein the circumferential raceway has a surface roughness Ra less than or equal to 18 microinchs without the application of any finishing process to the formed shell.
 20. A rolling bearing assembly manufactured by the process comprising the steps of: providing a cold-rolled strip having a carbon content greater than 0.65% and a plain strain forming limit value (FL₀) greater than or equal to 0.25; forming the strip to form a shell having a circumferential raceway; and positioning a plurality of rolling elements in the shell. 